Hub and freewheel question
 

I have a pretty nice old wheel (campy tipo hubs, super champion 58 rims, DT spokes) with 126 mm spacing and a 5 speed freewheel. The wheel was built for a 5 speed freewheel but I want to replace the 5 speed freewheel with a 7 speed one.

I figure I'll have to remove a spacer or two from the non freewheel side and add it to the freewheel side so that the cone locknut can reach the frame drop out through the wider freewheel (I measured the freewheels and the difference is right around 3 mm).

Once I do this, I'll obviously have to redish the freewheel.

My question is I think there is no magic formula for how many mm of spacing I need on the freewheel side, right? I think the only things I need to worry about is whether the cone locknut can reach the frame drop out and whether there is enough clearance so that there is no chain rub once I'm done.

Is this pretty much all there is to it or am I missing something here?

You pretty much have the idea. But likely the LH spacers are only a thin "lock" washer. Maybe 1mm. By test fitting the freewheel to the hub and then measuring how much more RH spacing you need to get the end cog (and chain as it lifts up to shift onto and off this cog) to clear the frame. Then you can determine if there is enough axle to allow for this longer spacing. Don't be surprised if you nearly run out of axle beyond the lock nuts.

Having done this many times I find it easier to just replace the axle with a longer one and add spacers as needed without worrying about the axle being long enough. Less frustrating to make the likely choices up front. I also would spread the frame to 130 and add spacers on the LH side after the RH side was figured to get to a 130 hub width. Makes for less wheel dish and easier future changes. Andy.

As you put more spacers on the freewheel side, you will increase the chance of bending the axle also.
If you're 1970's light, you will be mostly OK.